1. Field of the Invention
The present invention relates to an apparatus for receiving a signal conveying analog luminance and chrominance information components associated with coded digital signal periods, comprising a d.c. component recovery circuit and a decoder circuit for decoding the received digital signal. Such a signal is more specifically a signal of the MAC type, which comprises analog luminance and chrominance information components and digital information components coded per packet as well as analog reference voltage plateaus of the average grey level.
Such an apparatus is, for example, a television set arranged for receiving a signal in the D2-MAC/packet mode, or a satellite tuner or even any other apparatus having a member effecting a processing operation of the signal in a transmission system.
2. Description of the Related Art
Such an apparatus is disclosed in, for example, French Patent Application FR-A-2, 610,773. As described in that document, a television set is provided with means for re-establishing the d.c. component, in different successive stages which are separate from each other, first on the basis of the peak levels measured over the overall signal, then on the basis of the peak levels which are measured only on that part of the signal which is digital, and finally on the basis of the reference voltage plateaus of the signal.
The problem to be solved stems from the fact that the so-called "dc" component, which also comprises a low-frequency component, is negatively influenced by the disturbances which are inherent to all types of transmission systems, whether they are correlated or not correlated with the television signal. These disturbances constitute a parasitic signal comprising two components, i.e. noise having a frequency which is lower than to the line frequency (low-frequency break, energy dispersion, 50 Hz residue, etc. . . . ), and modulation noise whose spectral power distribution depends on the type of transmission.
In the television set described in said document, these levels are subject, when peak levels are used which are measured over the digital part to the signal, of varying versus the digital content of the signal, and the recovery of the d.c. component will not be accurate, and when the reference plateau is used to extract this component, one meets with the theoretical limits of recovery of a signal in the noise. Actually, the plateau duration, which is very short, does not permit of a sufficient integration of one line. Simple filters have been proposed which integrate the information components of several lines. Based on the principle of a first-order recursive filter, they have the advantage of reducing the noise in the case of unfavorable transmission types, but they lead to a measuring error of the low-frequency component. The trials effected for a transmission of a television signal in accordance with the D2-MAC/packet standard have shown how difficult it is to obtain a compromise between noise and low-frequency residues, which compromise must be adapted to each specific case constituting each modulation type.